One efficient semiconductor heterostructure electro-optical conversion device is a GaAs compound semiconductor having a pn junction therein and having a region of a graded band gap of GaAlAs on the surface thereof. Such a structure is shown in U.S. Pat. No. 4,122,476.
Greater efficiency in the GaAs-GaAlAs type structures was achieved where a graded band gap surface layer was provided consisting of AlGaAs which is graded from 0 to the vicinity of 80-100% aluminum over an approximately 200 nanometer depth from the surface. This provides a large band gap surface layer which enhances the short wavelength photoresponse. The surface layer is usually as heavily doped as possible to minimize an inactive region adjacent the surface that is depleted of carriers and in which there is no useful electro-optical conversion. Such structures are generally manufactured by the technique of molecular beam epitaxy using beryllium as a p-type dopant as described in a 1985 Electronic Materials Conference paper by J. M. Woodall entitled "A p.sup.+ /p/n GaAlAs/GaAs/GaAs Graded Gap Solar Cell Structure with Improved UV Response".
In semiconductor injection laser technology, efforts have been directed to preventing the heat of the device from oxidizing the light emitting surface (IEEE Spectrum, June 1985, pages 43-53, "Laser diodes are power-packed" by Dan Botez).
As more potential applications for these devices are encountered, a tendency of the surface to react to the environment usually by oxidizing adversely affects performance. Heretofore, accommodations built into the structure for environmental reasons often conflict with other optical and electrical requirements.